I ran a 6 volt starter for years with a 12 volt system. As long as you don't have to crank it for a long period of time, it will function correctly. If you crank for a long time, they will overheat and the armature will expand causing the starter to drag.
If you change to a 12 volt starter, you will also have to change the flywheel to have the appropriate number of teeth to function with a 12 volt starter. The 12 volt starter teeth look the same and look like they will match up, but they will not.
Joe
